Hydrothermally differential Ni–Mo mineralization in black shales and feedback on ocean oxygenation during the peak Cambrian explosion

Abstract The Cambrian explosion marks a key period in Earth's evolution, with the first appearance of nearly all known modern metazoan phyla and associated pronounced Earth system changes. The nickel–molybdenum (Ni–Mo) sulfide-rich polymetallic black shales in the 521 Ma Niutitang Formation in the South China block are unparalleled worldwide and therefore of significance for economic geology and Earth system science research. However, their formation regimes and broader implications are not fully understood. Based on high-resolution petrological and geochemical analyses, combined with available data sets, we found that the ocean shifted from oxygen-rich to hydrogen sulfide–rich (euxinic) between Cambrian Age 2 and Age 3, conditions which, along with hydrothermal metal input, promoted the formation of metal sulfides. Hydrothermally sourced Ni precipitated directly, whereas Mo was concentrated via seawater circulation and redeposition in euxinic seawater. The formation and burial of organic matter and sulfides enhanced net oxygen release to the atmosphere, contributing to the subsequent shallow-ocean oxygenation and further setting the stage for the third (main) phase of the Cambrian explosion. Our findings highlight the interplay of hydrothermalism, ocean redox evolution, and biological innovation during the Cambrian explosion, and explain the uniqueness of these mineralized shales with different patterns in that key period.